Unit test support for app engine.  Currently very limited as most of
our tests depend on direct network access, but these tests ensure that the
modules that are supposed to work on app engine don't depend on any
forbidden modules.

The code lives in maint/appengine/common, but should be run from the py25
or py27 subdirectories (which contain an app.yaml and a bunch of symlinks).
runtests.py is the entry point; cgi_runtests.py is used internally.